1. Field of the Invention
The present invention relates to a multi-layered strain sensor with a resistor fitted therein, in particular to such a strain sensor in which the degree of strain in the elastic region of its metal substrate is increased while the residual strain in that region is reduced to thereby enable accurate strain detection in a broad detectable range.
2. Description of the Related Art
Ordinary strain sensors generally used in the art have a multi-layered structure that comprises an insulating layer of a glass material formed on the surface of a metal substrate, a pair of electrodes formed on the surface of the insulating layer, and a resistor formed between the electrodes.
In the strain sensor of the type, when strain is imparted to the metal substrate in the tensile direction or in the compressive direction, both the metal substrate and the resistor receive the strain and the electric resistance value of the resistor is thereby changed. The resistor is packaged, for example, in a detection circuit such as a Wheatstone bridge circuit, and its electric resistance change is detected. With that, the strain sensor gives a detection output that depends on the degree of strain imparted thereto.
For example, the metal substrate of the strain sensor is generally made of SUS430 (18Cr stainless).
The strain sensor is to detect the electric resistance change of the resistor therein when some strain acceptable in its elastic region is imparted thereto. Accordingly, for broadening the strain-detectable range of the strain sensor, it is desirable that the strain range (up to the yield point) in the elastic region of the metal substrate of the strain sensor is broadened.
However, when the metal substrate is formed of a SUS material, for example, ordinary ferrite-type stainless steel such as SUS430 or austenite-type stainless steel, it has a drawback in that the strain range acceptable in its elastic region is narrow.
One reason of the drawback is as follows: When an insulating film of a glass material is formed on the metal substrate, its material is baked thereon, and, in addition, when electrodes and a resistor of a cermet material or the like are formed thereon, their materials are also baked. The baking is effected at high temperatures of from 800 to 900xc2x0 C. As a result, the metal substrate formed of stainless steel such as SUS430 becomes brittle through such high-temperature baking treatment and its yield point is thereby lowered.
In addition, when the ferrite stainless steel such as SUS430 is processed at such high temperatures and when the thus-processed stainless steel receives strain in its elastic region, it has residual strain therein. In that condition, when the stainless steel thus having residual strain therein further receives strain and is thereafter restored to its original condition owing to its elasticity, it still keeps the residual strain therein. As a result, when the stainless steel of the type is used in a strain sensor for detecting an electric resistance value, the accuracy of the sensor to be restored to its starting point is low and the sensor will fail to attain accurate detection.
The present invention is to solve the related-art problems noted above, and its one object is to provide a strain sensor, which is specifically so designed that the strain range of the metal substrate thereof is broadened in its elastic region to thereby ensure a broadened strain-detectable region of the sensor, and to provide a method for fabricating the strain sensor.
Another object of the invention is to provide a strain sensor of which the residual strain remaining in its elastic region is reduced to thereby enable accurate strain detection with it, and to provide a method for fabricating the strain sensor.
The invention provides a strain sensor which comprises a metal substrate, an insulating layer formed on the surface of the metal substrate, a resistor formed on the surface of the insulating layer, and electrodes connected to both ends of the resistor, respectively;
wherein the metal substrate is formed of a stainless steel that contains from 16 to 26% by weight of Cr and from 2 to 6% by weight of Mo.
The stainless steel that contains Cr and Mo has a high tensile strength and its yield point and hardness are also both high, and therefore its strain range in its elastic region is broad.
In the invention, the stainless steel may be of a type of ferrite, but is preferably of a type of two-phase configuration that contains from 3 to 8% by weight of Ni. Specifically, it is preferable that the metal substrate is formed of such a two-phase stainless steel that contains from 16 to 26% by weight of Cr, from 3 to 8% by weight of Ni and from 2 to 6% by weight of Mo. The two-phase stainless steel of the type has a high tensile strength and a high yield point, and therefore has a broad strain range in its elastic region. Accordingly, it broadens the strain-detectable range of the strain sensor comprising it.
Also preferably, the metal configuration of the stainless steel contains a sigma phase, or contains both a gamma phase and a sigma phase. The insulating layer is made of, for example, a glass material.
The method for producing the strain sensor of the invention comprises:
(a) a step of forming a layer of a pasty glass material on the surface of a metal substrate of a stainless steel that contains from 16 to 26% by weight of Cr and from 2 to 6% by weight of Mo,
(b) a step of baking the pasty glass material to form an insulating layer,
(c) a step of forming electrodes and a resistor on the insulating layer through baking their materials;
wherein the baking temperature is not lower than 800xc2x0 C. in at least any one baking step of (b) and (c).
Preferably in the method, the metal substrate is formed of a stainless steel that contains from 16 to 26% by weight of Cr, from 3 to 8% by weight of Ni and from 2 to 6% by weight of Mo.
In the invention, the stainless steel containing Mo is use. Therefore, when it is baked at a temperature not lower than 800xc2x0 C., its alpha phase changes into a sigma phase and therefore both its hardness and yield point increase. As compared with Mo-free stainless steel heated at such a temperature, the Mo-containing stainless steel for use in the invention can broaden its strain range in its elastic region and can reduce its residual strain in its elastic region.